The existence of inhibin as a water-soluble substance of gonadal origin which acts specifically at the pituitary level to suppress the secretion of follicle-stimulating hormone (FSH) was postulated by McCullagh more than 50 years ago, Science, 76, 19-20 (1932). There has been great interest in it, and many laboratories have attempted to isolate and characterize this substance. Many reports have appeared in the literature claiming the purification of inhibin-like material; however subsequent studies have shown that these substances were either not homogenous or did not have the high specific activity of true inhibin. Inhibin may be used to regulate fertility, gonadotropin secretion or sex hormone production in mammalians, both females and particularly males.